Silicon on insulator ("SOI") technology deals with the formation of transistors in a layer of semiconductor material known as a mesa which overlies an insulating layer. The most common embodiment of SOI structures is a single crystal layer of silicon which overlies a layer of silicon dioxide. High performance and high density integrated circuits are achievable using SOI technology due to the reduction of parasitic elements as compared to non-SOI technology formed in bulk semiconductor. For example, for a MOS transistor formed in bulk, parasitic capacitance is present at the junction between the source/drain regions and the underlying substrate. Further, the possibility of breakdown of the junction exists between the source/drain regions and the substrate region. Still further, in CMOS technology in bulk, parasitic bipolar transistors are formed by N channel and P channel transistors in adjacent wells and can give rise to latch-up problems. SOI structures significantly alleviate the parasitic elements and increase junction breakdown tolerance. Thus, SOI technology is well suited for high performance and high density integrated circuits.
In bulk transistors, electrical connection is made via the substrate to the body node of a MOS transistor. The relatively fixed bias of the body node resulting from this connection provides for a stable threshold voltage relative to the drain-to-source voltage. Comparatively, conventional SOI transistors provide a body node which is electrically floating as the body node is isolated from the substrate by the underlying insulator. Under sufficient drain-to-source bias, impact ionization can generate electron hole pairs near the drain which, due to the majority carriers traveling to the body node while the minority carriers travel to the drain, cause a voltage differential between the body node and the source of the transistor. This voltage differential lowers the effective threshold voltage and increases the drain current, thereby exhibiting the well-known "kink " effect.
The floating body node of the SOI transistor also presents a parasitic back-channel transistor with the substrate as the gate and the insulator underlying the transistor as the gate dielectric. This back-channel may provide for a drain-to-source leakage path along the body node near the interface with the underlying insulator. In addition, the dielectrically isolated body node allows capacitive coupling between the body node and the gate and diode coupling between the body node and the source and drain. Either of these coupling effects may cause the body node to be biased and thus affect the threshold voltage of the SOI transistor. Each of these factors can contribute to undesirable performance shifts in the transistors relative to design, as well as to increased instability of the transistor operating characteristics.
In light of the aforesaid, it is thus desirable to fix the voltage of the body node rather than allowing it to float. Typically, the body node is kept at the same voltage as the source or may be controlled at a voltage different therefrom. In devices heretofore known, control of the body node voltage is effected by either contacting the body node directly at the edge of the transistor or, alternatively, providing a semiconductor contact region at the surface of the transistor which may be contacted to control the voltage of the body node. However, these existing techniques for contacting the body node provide accurate control voltage only at points immediately proximate the contact. The resistance to any point of the body node increases with distance away from the contact. Therefore, any current provided through the contact will cause a corresponding increase in voltage across the body node at distances away from the contact. As a result, a single contact does not provide a uniform voltage across the entire body node. A nonuniform voltage along the body node will degrade the operating characteristics of the transistor. The doping, and thus the conductivity of the body node, is limited by restrictions to obtain the desired transistor characteristics, such as threshold voltage.
Therefore, a need has arisen for a body node contact which may control the voltage bias to the body node of a SOI transistor in a uniform fashion across the entire length thereof. There also exists a need to uniformly control the body node voltage independent of the source voltage.